Communication networks are common. Most communication networks experience degradation in transmitted signals. This degradation may be from signal loss directly, such as smearing of the signal through the medium, loss of signal strength, etc. Another source of degradation is noise. Noise may be wideband, narrowband, Gaussian, colored, etc. Another source of signal degradation may be from other signals. Often this type of degradation or interference is called crosstalk (also cross-talk).
Crosstalk refers to the case signals become superimposed upon each other. The signals may be superimposed by electromagnetic (inductive) and/or electrostatic (capacitive) coupling in wireline networks. Signals from adjacent transmitters may also be superimposed over the air in wireless networks. Also, signals from adjacent frequency bands or wavelengths may be superimposed in cable and optical networks respectively. Crosstalk may come from a variety of physical sources and/or properties, such as bundles of twisted pairs that may be capacitively coupled. In bundles of wires, crosstalk may be reduced by the use of shielded cables or increasing the distance between the signal carrying lines. In wireless and optical networks, crosstalk may be reduced by increasing the transmitter and wavelength spacing respectively. Shielded cables are more expensive than twisted pair and so this results in increased cost. Increasing the distance between conductors would result in an increased cable bundle size that may present a space problem. Similarly, increasing the distance between transmitters or wavelengths in wireless and optical networks reduces the system's capacity. Thus, signal crosstalk is a problem because it degrades communications. For this reason, the accurate characterization of the interfering sources may be useful in the analysis, diagnosis and ultimately mitigation of the interference.